An electromagnetic field impinging on an antenna can be resolved into two orthogonal polarized components. For example, one electric field component may be aligned perpendicular to the earth's surface (i.e., vertically polarized), while the other electric field component runs parallel to the earth's surface (i.e., is horizontally polarized). These field components are received by orthogonally polarized antennas (e.g., a vertical dipole and a horizontal dipole) to produce corresponding signals whose relative phase and amplitude are a function of the incident field's polarization.
In many applications, it is desirable to combine these signals for processing or analysis. Such applications include signal enhancement, interference suppression, target signal analysis, adjacent channel separation, wind shear detection, and multipath reduction. A circuit, known as a polarization combiner, is provided to combine the signals, which, as noted, may be of arbitrary amplitude and phase. Preferably, the signals produced in response to the orthogonally polarized field components are combined into a single output exhibiting the total power of the two signals and an optimum signal-to-noise ratio.
As will be appreciated, the incident polarization of the field received by the antennas connected to the polarization combiner may change significantly with time. For example, the polarization may be affected by changes in the relative orientation of the transmitter and receiver or changes in the transmission medium. In any event, the polarization combiner must respond to such changes in the incident polarization, maintaining the total power and optimum signal-to-noise ratio at the output.
In that regard, polarization combiners have been developed that respond to changes in incident field polarization. For example, in an open-loop arrangement, the processing of signals corresponding to orthogonally polarized field components is manually adjusted to achieve optimum performance. The adjustments are made in response to information gained by monitoring the power of the output signal or by having some a priori knowledge of the incident field polarization. As will be appreciated, however, the primary disadvantage of such an approach is the slow response of the open loop to sudden changes in the incident polarization.
To improve the response time, an automatic, closed-loop polarization combiner has also been developed. As disclosed in U.S. Pat. No. 3,310,805 (Viglietta et al.), such a combiner includes two hybrid junctions, each of which is preceded by a variable phase shifter. This circuit receives the orthogonally polarized components of the incident field from two orthogonally polarized antennas and produces a maximized summation signal and a minimized "difference" or "null" signal, which are used to provide feedback to the variable phase shifters. More particularly, couplers provide the summation and difference signals to a pair of phase discriminators. These phase discriminators perform the scalar or dot product of the vector summation signal and the vector difference signal, as well as the dot product of the summation signal and the quadrature of the difference signal. The outputs of the phase discriminators are applied to servodrive mechanisms, which control the variable phase shifters to accomplish adaptive polarization combining.
As will be appreciated, with the polarization combiner properly adjusted, the summation signal will be maximized and the difference signal will be zero. In that case, the output of both phase discriminators will be zero and no adjustment will be made to the variable phase shifters. In the event a nonzero difference signal is produced, however, the phase discriminators will produce outputs that are proportional to the components of the difference signal that are in-phase and in-quadrature with the summation signal. In response, the servodrive mechanisms cause the variable phase shifters to restore the proper output conditions.
Conventional automatic polarization combiners of this type suffer several disadvantages. First, the manner in which the variable phase shifters can be adjusted is limited. In that regard, conventional phase shifters have input range limits that cannot be exceeded if proper operation is to occur. However, the phase shifters may be required to work outside these range limits if the polarization combiner is to accommodate any anticipated variations in incident field polarization. Thus, although this problem has not been addressed by the prior art, when the phase shifter reaches its operating limit, it must be able to effectively shift between that limit and a point in its operating range that is some multiple of 360 degrees away from the limit.
In addition, prior automatic polarization combiners do not work properly when one of the orthogonally polarized components of the incident field is not present. In this situation, the corresponding input signal is absent and the desired summation and null signals should be produced without adjustment to the phase shifters. The presence of circuit component imperfections, however, will likely cause the servodrive mechanisms to attempt to adjust the phase shifters. Because one of the signals is absent, there is no signal for the phase shifter to process, to accomplish the desired adjustment. As a result, the combiner would dither.
In view of these observations, it would be desirable to provide an adaptive polarization combiner that has a fast response time, is relatively simple in construction, properly responds when one of the input signals is absent, and can employ phase shifters subject to operating limits without impairing the combiner's ability to respond to changes in the incident field's polarization.